Circadian clocks are endogenous oscillators that drive daily rhythms of biological processes. In mammals, circadian clocks are found in the brain and in most peripheral tissues. Distributed clocks appear to constitute a fundamental timing system that coordinates behavior and physiology. The mammalian clock is built from a transcriptional feedback loop that generates circadian rhythms at the molecular level. The dimeric transcription factor CLOCK- BMAL1 is at the heart of this feedback loop, but surprisingly little is known about its mechanism of transcriptional action or the specific processes causing its negative feedback suppression, arguably the defining feature of the clock mechanism. This application proposes biochemical purification and mass spectrometry to identify, comprehensively if possible, the proteins in a complex with BMAL1. Candidates from mass spectrometry will be tested for co-immunoprecipitation with BMAL1 from multiple tissues. Confirmed BMAL1-associated proteins will then be studied in established paradigms, including loss-of-function studies, to determine if they play a role in circadian oscillations and, if so, the nature of that role in molecular terms. It is likely that many of the protein constituents of BMAL1 complexes have known functions, potentially offering novel clues to molecular events at the core of the clock. The project aims to identify proteins playing an important but unrecognized role in the circadian feedback loop, either in CLOCK-BMAL1 transcriptional action or in feedback suppression of CLOCK-BMAL1 transcriptional activity. If the aims are achieved, the project has the potential to expand and deepen our knowledge of the mammalian clock, possibly in unanticipated directions. Advances in understanding the mammalian circadian clock will have important implications for our view of the genetic control of behavior and physiology, as well as for human health and disease. Mouse genetic studies indicate that defects of clock function lead to broad behavioral and metabolic dysfunction, producing, for example, disrupted sleep-wake cycles, abnormal feeding behavior, and a metabolic syndrome like early-stage diabetes. If successful, the proposed study could provide new insight into genetic and molecular mechanisms controlling behavioral and physiological programs.